using chemical weapons against their own people but suffice it to say we are certainly planning to take action if that eventuality were to occur. >> reporter: seizing the chemical weapons and agents would require putting up to 75,000 troops on the ground. a massive operation that u.s. officials say is only likely to happen if and when assad falls. preventing assad from using his weapons would require air strikes against sights defended by russian-made anti-aircraft missiles. for now, the obama administration is backing up its public warnings by sending private messages to assad through one of his few remaining allies, russia. >> pelley: david, thank you. the assad dictatorship has ruled syria for more than 40 years. the syrian people rebelled last year and it's been open warfare ever since. it's extremely hazardous for reporters to get into the war zone, but elizabeth palmer managed to reach a neighborhood under siege on the outskirts of the capital city, damascus. >> reporter: it's 6:00 p.m. the heavy shelling doesn't usually start until 9:00, so it's safe to go for a tour of this wrecked and